When is the best time of year to travel on Amtrak's Calif. Zephyr from Emeryville to Denver.?

We want to see the best scenery, wildlife, river views, etc. Should we try to make a stop at Salt Lake City? Go to Yellowstone? We can go any time of year and want to experience an overnight train trip and this seems like it should be a good choice. If anyone has a better suggestion, we will certainly consider it.